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A308449 Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime. 1
0, 1, 2, 14, 174, 448 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

31 is prime ==> a(1) = 0.

2551 is prime ==> a(2) = 1.

250501 is prime ==> a(3) = 2.

25005001 = 7 * 79 * 103 * 439.

2500050001 = 19 * 2269 * 57991.

250000500001 = 7 * 103561 * 344863.

PROG

(PARI) for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", ")))

CROSSREFS

Cf. A309738.

Sequence in context: A235369 A351274 A228476 * A233224 A167014 A300282

Adjacent sequences: A308446 A308447 A308448 * A308450 A308451 A308452

KEYWORD

nonn,more,base

AUTHOR

Seiichi Manyama, Aug 15 2019

STATUS

approved

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Last modified December 6 05:22 EST 2022. Contains 358594 sequences. (Running on oeis4.)